“Where there is an observatory and a telescope, we expect that any eyes will see new worlds at once.” -Henry David Thoreau
Oh, let’s be real. While there was plenty to talk about here at Starts With A Bang, there was one thing that took over the news from everything else, the first ever discovery of gravitational waves! Sure, there were plenty of other remarkable stories, including:
- Are we due for an extinction event on Earth? (for Ask Ethan),
- Why do telescopes have holes in the middle? (for Mostly Mute Monday),
- What will it mean if LIGO detects gravitational waves?,
- The future of astronomy: the starshade and exoplanet imaging, (a new Future Of Astronomy post)
- The first detection of gravitational waves, and
- The physics of the perfect chocolate for Valentine’s Day.
For those of you who didn’t catch it, the discovery of gravitational waves was so big that I was on the news talking about them:
Now, with the formalities out of the way, let’s see what you’ve had to say for our Comments of the Week!
From Omega Centauri on extinction events: “I’m not fully up to date on extinction event causes, but I have the impression that impacts are only implicated in one or two of them.”
This is very, very much the case. Some non-extraterrestrial-impact causes include the great oxygenation event, where microbial life poisoned its own atmosphere and nearly froze the world to death, theorized massive eruption/supervolcano events and the current mass extinction, which is entirely human-caused. In addition, despite the evidence of Chixulub crater and the KT boundary layer, the next best candidate (that possibly caused the great Permian extinction) shows evidence of a great crater in Antarctica.
The Wilkes Land Crater, if caused by an asteroid impact, would have been five to ten times as powerful as the dinosaur-extincting asteroid from 65 million years ago. However, there is no analogue to the KT layer in the sedimentary rock record, leading many to theorize that even that was not a giant impactor, but rather a different geophysical feature. They say that prediction is difficult, especially about the future, but trying to reconstruct the past from a scant series of mostly missing clues is pretty hard, too!
From G on what ifs: “If our star system passes through a higher-risk region of our galaxy every 26 – 30 million years, then we should be able to roughly predict the time until the next such passage.”
Does it, though? Is passing through the galactic plane a quantifiably riskier proposition than not? I don’t think it is at all. Passing through the spiral arms is likely to be a riskier proposition, but nobody talks about that. Yes, when things pass by and disturb the Oort cloud (or even the Kuiper belt), that’s potentially hazardous to us! But is that risk elevated by passing through the galactic plane?
Signs point to no. It’s important not to build a house-of-cards on an invalid premise, and that’s exactly what the notion that extinction events are periodic and due to asteroid impacts is. Get some data that proves this isn’t “Garbage-In,” and then (and only then) will we talk about what we get out.
From PJ on offset vs. non-offset telescopes: “When discussing professional telescopes of such large apertures and small focal ratios (f4 to f7), of course your comments are totally valid. For amateurs (backyarders) we do not have the luxury of such projects. The average user would be lucky to have an aperture of 8 inches or so, extending to, maybe 16. For wide field viewing, as in nebulae & extended objects, the wide field approach is preferred (fields of 2 to 3 degrees or so). When it comes to lunar & planetary, we start to look at a narrower FOV. This is where longer focal lengths come into play (f10 to f15). Using offset optics give maximum light gathering for such modest apertures and alleviate any inherrent artefacts from spider & secondary diffraction.
For the professional institutes, I could not imagine a VLT with a focal ratio of 10:1. Phew, blows the mind.”
I — a professional theorist but a very, very amateur observational astronomer (although I did once write an observational paper: hey!) — really spend almost all of my time doing wide-field viewing, either with binoculars or with my 10″ Dobsonian. Dobsonians are the least expensive way to get large-aperture views of the Universe, and wide-field eyepieces allow you to see more of the sky than anything else. But if you want to see things like individual planets, you’re better off with high magnifications, and often with alternative designs than the simplest ones.
When PJ talks about “focal length,” he means higher magnifications and narrower fields of view, as “long” focal length is a measure of how long a distance it takes to converge your light. The focal lengths on the Keck telescopes, by the way, are 17.5 meters, or f/1.75. If you wanted f/10 or f/15, you’d be talking optical systems that were football-field scales. Good luck fitting that inside a dome!
From Denier on gravitational waves: “If gravitational waves are detected by LIGO, doesn’t that mean theoretical models that quantize space-time (like LQG) are incorrect?”
Not at all! What appears as ripples of radiation in General Relativity is identical to a spin-2, massless graviton traveling at the speed of light, and so it’s the other way around: if there were no gravitational waves, that would mean quantum gravity was incorrect. One of the interesting things that came out of this was that, based on the properties of the ripples that LIGO did see, we can constrain that if the graviton does have a mass, it’s less than ~10^-22 eV, or about 10^28 times lighter than the electron. This is the best limit on the masslessness of the graviton ever, which further confirms our idea that, at some level, gravitation must be a quantum theory.
From Ragtag Media off-topic but on the great attractor: “Can you good people direct me to some more authoritative reading on this ”Great Attractor”?”
There were some basic links provided, so here’s my run-through, as quickly as possible:
- You can estimate the masses and positions of everything in our nearby Universe, and calculate what their gravitational pulls are on everything else.
- When you do this, you find that the cosmic motions we observe are “off” by quite a bit: there appears to be an additional, unexplained peculiar velocity flow towards a region of sky in the constellation of Centaurus.
- Because we don’t see the mass but assume it must be there, we’ve given it a name: the Great Attractor.
It was assumed that one of a number of clusters (including, at one point, the Centaurus cluster) was the cause of this, but in 2000 there was a paper by a team of seven astronomers (awesomely dubbed the “Seven Samurai”) that showed the origin of the peculiar velocity was unknown. We are still searching for this cause and this mass, and there’s a good chance that it’s aligned with the plane of our own galaxy, which we have extraordinary difficulty seeing through. So that’s where we are today on the Great Attractor story.
From Paul Dekous on LIGO’s discovery: “It is clear that he measured a vibration on earth, but I find it a little strange that he knows straight away and exactly that it are 2 black holes with clearly defined masses such and such, and all this from a 1st observation.”
This is exactly why theorists calculate models! The lines you see in the bottom panels that are labeled with “numerical relativity” are exactly these calculations. The models have been calculated for all sorts of different mass and orbital possibilities, and the fact that what we saw matched what we predicted told us:
- The mass of each progenitor black hole.
- The timescale/timetable of each orbit during the merger.
- The amount of energy released during the merger.
- The final mass of the end-state black hole.
- And — most importantly — it gave us a measure of how well the theory agreed with what we saw.
The last one turned out to be “perfectly, to the limit of our equipment.” You see, we already had the theory of this worked out: it just came about as a consequence of Einstein’s relativity. The rest was a lot of hard work, and what appears to you as “a little strange” is the result of decades of hard work by over a thousand individuals.
From Barzini out of left field: “How come we have no video of the entire earth spinning?”
Here you go:
From Sinisa Lazarek on the importance of the LIGO discovery: “what I don’t understand is how will this usher a new age in astronomy? What can we effectively “observe” that we couldn’t before? Anything about early universe seems too far away and too faint to get anything tangible.”
Okay, okay. I want you to imagine that for all of your life, it was overcast and cloudy. You never saw the Sun, the Moon, the stars, a planet, or any patch of sky. Just clouds. Then one night, the clouds cleared, and you saw your first object. It happened to be a planet, and you didn’t just see it, you saw it so well that you realized it had rings, satellites, bands on it and more. And then it clouded over again.
That’s what LIGO was like: for the first time, yes, we saw gravitational waves. But what we learned from them was something we have no other way of learning: we learned about black hole mergers! We learned how much mass gets turned into energy during one; we learned that the power emitted during a merger can (briefly) out-shine all the stars in the Universe; we learned that ripples in space obey Einstein’s GR. With other missions — like LISA or BBO — we’ll be able to learn even more. With more stations like VIRGO and CLIO, we’ll hone in on the position of these objects even better. It’s an incredible time for astronomy, and an incredible time to be alive.
From Adam on black holes and information: “If people are worried about information paradox as it relates to the immensely slow rate of Hawking Radiation from a black hole (what was it, one photon lost in a trillion years?), how does it relate to losing 3 solar masses worth of energy in 20 milliseconds?”
What information is encoded in gravitational radiation? That’s the real question, and until we can measure it better — which will take a large improvement in our technology and our understanding — it’s pretty hard to answer that question! But yes, people should worry.
From Michael Kelsey, who’s often a deluge of useful information: “There are several points which address the naive and mispaced skepticism in these comments.
1) The signal amplitude (that is, the dimensionless strain) depends only on the distance to the source (and it falls like 1/D, rather than 1/D^2), not on the object masses.
2) The frequency leading up to the inspiral is just twice the orbital frequency, and depends on the two masses. There is a power-law relation which connects the product and sum of the masses.
3) The rate of inspiral, and hence how the frequency changes during the signal, also depends on the two masses, but in a different way. Combined with (2), this allows extraction of both masses individually.
4) The frequency _after_ the merger (the “ringdown” frequency) depends only on the final mass, and is an independent measurement from (2) and (3).
5) The peak frequency at the end of the inspiral tells you directly how far apart the two objects were. Combine that distance with the masses from (2) and (3), and even an ingornantly skeptical ass-hat can compute the densities of the two objects, and compare that density with, for example, stars, planets, or neutron stars. I leave it as an exercise for the reader to do that trivial calculation for the case of 30 solar masses and a minimum separation of about 350 km, and to report what kinds of objects might have that density.
6) The specific relationships for (2), (3) and (4), along with the detailed shape of the waveform, also depend on the black hole spins, and how they align with the orbit. Those details allow information about the spins to be extracted by comparing the measured waveform to computed templates.”
From Rick on the merger: “Was a short gamma ray burst associated with this merger?”
I wish, but the best answer is maybe! Unfortunately, the primary cause of short gamma ray bursts are thought to be neutron star mergers, which are out of the mass (and hence, frequency) range for LIGO; the causes of long GRBs is thought to be supernova events, which might be out of LIGO’s amplitude range. Black Hole mergers emit lots of energy, but it’s expected to be in the form of gravitational radiation. GRBs could arise, though, from the material around the black holes during the merger; they’d be expected to be very weak. Such a signal was in fact noticed by the Fermi satellite, as Paul Dekous points out, but that signal is not as robust as we’d hope for.
I will remind everyone, however, that the expected event rate for this type of merger is that LIGO should see between 2-5 per year of ~20-to-50 solar mass black holes merging with other 20-to-50 solar mass black holes, to say nothing of the other events it hopes to see. Please, please stay tuned!
And finally, from Ragtag Media on chocolate: “You want a good Chocolate story, go read up on that great (sic) America .Milton S. Hershey who founded the Hershey company.”
I have been to Hershey, PA, and I’ve been on that tour! (They only gave out Hershey’s kisses at the end, FYI, as of 1990-whatever when I went.) However, I will point out that Hershey’s chocolate no longer contains enough real chocolate to be useful in the chocolate making process I detailed. Wax and fillers are your enemy; cocoa butter and cocoa solids are your friends, along with sugar and (perhaps) milk, if that’s the way you roll. That’s it!
Thanks for a great week, everyone, and see you back tomorrow for more wonders of the Universe!